Add Levenshtein Distance function to bridgedb.util module.

 * ADD new function, `bridgedb.util.levenshteinDistance()`.
Add EMAIL_BLACKLIST and EMAIL_FUZZY_MATCH settings.

 * ADD the EMAIL_BLACKLIST setting, which is a list of blacklisted
   (spammer/harvester) email addresses.

 * ADD the EMAIL_FUZZY_MATCH setting, which allows for fuzzy matching of the
   EMAIL_BLACKLIST addresses via the Levenshtein Distance algorithm.
Fuzzy match incoming email addresses.

 * ADD a branch to `bridgedb.email.autoresponder.SMTPAutoresponder.runChecks()`
   which calculates the Levenshtein Distance of an incoming email address in
   order to fuzzy match it against those in the EMAIL_BLACKLIST setting.

 * FIXES #9385:
   https://trac.torproject.org/projects/tor/ticket/9385
Add EMAIL_BLACKLIST and EMAIL_FUZZY_MATCH settings to test/email_helpers.py.
Add unittests for bridgedb.util.levenshteinDistance().
Add unittest for SMTPAutoresponder.runChecks() with blacklisted email.
Merge branch 'fix/9385-fuzzy-email-matching_r1' into develop